This invention relates to outboard motors and inboard-outboard motors and more specifically to the underwater exhaust systems built into both of these types of marine propulsion particularly as used for fishing. It is well known that low frequency underwater sounds can be detected for great distances through water while sounds of higher frequencies attenuate rapidly and can be detected for only relatively short distances. Marine biologists have established that all fish are capable of hearing and that there is high sensitivity in regard to extremely low frequency sounds which make up the predominant portion of all underwater noises. The fish utilizes this acute hearing to carry on its normal living functions including protection of prey and awareness of potential enemies. All running powerboats radiate underwater noise similar in frequency to natural underwater noise and, in most cases, of considerably greater magnitude. This noise comes from three main sources. One is machinery noise--that generated by the motors. Another is cavitation noise produced by the propellers. The third is exhaust noise which becomes evident only when the exhaust is discharged below the water surface and, in this case of considerably greater magnitude than the machinery noise. During low speed operation, cavitation noise is not generated. Machinery noise can be reduced to acceptable intensity levels by sound insulating materials. Underwater exhaust noise, which is the predominant irritant to fish during low power operation, cannot be directly reduced in intensity but can be eliminated by means of an air exhaust by-pass as hereinafter described in accordance with the present invention.
The early outboard motors were low powered and designed to clamp to the stern of rowboats. They were a very acceptable substitute for rowing and enhanced the most effective method of fishing, which is trolling, by extending the territory which could be covered by several times. These early outboards had air exhausts and were noisy for passengers when run at even intermediate speeds. At trolling speeds the passenger noise level was not high and the number of fish striking was nearly the same as when rowing. Daily catches were higher because the fishing time was longer. Previously, the rower's back or arms would give out after a short time. As outboards grew in size, the high speed motor noise increased to a point where passengers were made uncomfortable and persons on shore became annoyed. It was found that, by routing the motor exhaust through a tube below the water level and releasing it there, the motor noise was considerably lessened for both passengers and bystanders. Thus, the universally used underwater exhaust was developed. At the time, no studies of the effect that underwater noise had on marine ecology were made. When trolling produced far fewer fish, it was thought that the area had been "fished out" although adequate catches were still being made by casting or still fishing. Today, scuba divers and snorklers realize that outboards and inboard-outboards generate excessive underwater noise. It is, however, looked upon as a necessary evil inherent to an economical means of pursuing their sport.